You should get your amounts correct, its 50pips not 500. You don't want to sound like a liar in the event you file a complaint. The last digit on yen pairs is only a point, the 2nd last is a pip.
But yeah looks like you got screwed. I was in the same trade, my sl was 115.1, got filled at 114.94. Totally ridiculous how you got a stop higher than mine and got filled so much lower. I bet its a case of b-booking and lagging your execution.
After reading your post I was going to put an entry in my personal notes on FXpro for dodgy slippage but I noticed there was already a note there from 2012 saying 'market maker scammers' lol. Good to see my notes panned out (this is why you should research your broker beforehand).
Now as for what you can do, I suggest first filing a complaint against them in both your country's regulators and the FCA. See if they're willing to negotiate once you escalate. Check if they have a local branch in your home country, if they do you can get a lawyer to take legal action. Even if they don't, you should still have some legal recourse given you're both in the EU. Ask your local lawyers/consumer advice agencies.
Personally I'm always amazed at how many of these bucketshops are still around. it seems like every day I read about a broker ripping off their clients. Given how often it seems to happen, I'm surprised that I haven't heard about angry clients taking a baseball bat to their offices or hiring hitmen or the like. Anyway good luck with trying to get your money back.